Embodiments of the present disclosure relate to a power factor correction circuit, and particularly to a high efficiency power factor correction circuit that reduces electromagnetic interference (EMI) noise.
A power factor of an alternating current (AC) power system is defined as a ratio of real power for a load to apparent power. This power factor is usually referred to as PF. For example, a power factor of an AC power system having a sinusoidal current waveform and a sinusoidal voltage waveform is represented as a cosine of a phase angle (cos θ) between the current waveform and the voltage waveform. Voltage is multiplied by current to calculate power of a DC circuit, but to calculate power of an AC circuit a root-mean-square (RMS) value of a voltage and a current is multiplied by a factor cos θ. The power factor of the AC power system having a non-sinusoidal current or voltage waveform are affected by several factors, including a displacement factor related to the phase angle and a distortion factor related to a non-sinusoidal waveform.
Generally, real power (for example, watt) may be defined as power that does actual work, and reactive power may be defined as power required to generate a magnetic field (for example, power loss) so that the actual work can be performed, and apparent power may be defined as a total power required to generate the desired real power. The power factor of the AC power system may vary in a range between 0 and 1, where 1 indicates a pure resistive circuit with no reactive power loss. If the power factor of the AC power system is not 1, then either the current waveform lags the voltage waveform or vice versa. As a result, power loss occurs, and harmonic waves passing through the AC power system may be potentially generated and interfere with other devices.
A load connected to a power source rarely includes only a pure resistive component, but usually has a capacitive load and/or an inductive load. Accordingly, a reactive power loss usually occurs. When an AC voltage is converted to a DC voltage and the DC voltage is used, a smoothing capacitor is used to reduce the AC component, which causes a decrease in the power factor. In order to reduce reactive power loss, a number of types of power factor correction circuits that make an input voltage have nearly the same phase as an input current are used. In particular, in a device that converts an AC voltage into a DC voltage and uses the DC voltage, a scheme of improving a power factor using a voltage step-up converter, that is, a boost converter, is often used.
Therefore, it is possible to increase the power factor of the AC power system using a power factor correction (PFC) circuit.